1. Field of the Invention
The present invention relates to transmission rate changes in communications networks, for example mobile communications networks such as Wideband Code Division Multiple Access (W-CDMA) cellular networks.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows parts of a mobile communications network including user equipment UE which is in two-way communication with a base station BS. The base station BS may also be referred to as a node-B. The user equipment UE and base station BS have one or more uplink (or reverse link) channels for transmitting signals (user data and/or control signals) from the user equipment to the base station, and one or more downlink (or forward link) channels for transmitting signals (user data and/or control signals) from the base station to the user equipment. There may be separate data and control channels in one or both directions. In a cellular mobile communication system, a wide coverage area is provided by providing several base stations, each base station having a coverage area that partly overlaps with that of a neighbouring base station, so that at certain times a UE may be capable of receiving signals from two or more base stations.
The base station BS is also in two-way communication with a base station controller BSC of the network. The base station controller may also be referred to as a radio network controller (RNC). Usually, several base stations are in communication with the same base station controller. The base station controller BSC is in turn in two-way communication with a mobile switching centre MSC. The base station controller BSC serves to manage the radio resources of its connected base stations, for example by performing hand-off and allocating radio channels. The mobile switching centre MSC serves to provide switching functions and coordinates location registration and call delivery.
One W-CDMA mobile communications network currently under development by the 3rd Generation Partnership Project (3GPP) is referred to as a UTRA network. UTRA stands for UMTS Terrestrial Radio Access, and UMTS stands for Universal Mobile Telecommunication System (a third generation mobile telecommunications system). The proposed UTRA network is a Direct Sequence CDMA (DS-CDMA) network using frequency division duplexing (FDD). In such a FDD network, uplink and downlink channels are realised using different frequencies (spaced by 130 MHz), and a physical channel is identified by a code and a frequency.
The UTRA network is being developed to meet the growing demand for the use of wireless access for multimedia applications which demand much higher bandwidth than was previously necessary for low data rate applications such as voice data transmission. Such third-generation systems should typically be able to offer transmission rates of at least 144 kb/s (preferably 384 kb/s) for high-mobility users with wide-area coverage and 2 Mb/s for low-mobility users with local coverage. The data may be sent in the form of packets.
In addition to applications calling for high transmission rates, there will be a demand for the capability to use multiple services simultaneously so that, for example, a user could browse the Internet while receiving a file from a corporate Intranet server as a background process.
Since in a W-CDMA mobile communications network multiple radio channels can share the same frequency band, careful control of transmission power must be maintained to minimise interference problems. The problem of interference will inevitably increase not only as the number of users increases, but also as the use of high data rate services such as those mentioned above increases.
Power-efficient use of the available spectrum is facilitated in W-CDMA by the use of closed-loop power control techniques on certain channels. With such techniques, the transmission power used at a transmission site is controlled, based for example on the quality of the signal received at a reception site, so as to adjust the transmission power to the minimum level required to maintain an acceptable quality of service (QoS) at the reception site. For example, if the bit error rate of signals received on a particular channel by a UE from a BS is too high (due perhaps to a rise in the overall interference level in the network) for an acceptable quality of service, the UE would send power control commands to the BS which would cause the BS to increase its transmission power for that channel in an attempt to restore the quality of service.
Generally speaking, the use of a higher transmission rate leads to a greater chance of reception errors since the data bits contained within the signal are more closely spaced and are therefore more susceptible to interference and noise. A higher transmission rate therefore typically requires a greater transmission power to be used in order to overcome the increase in error rate.
As a result of this, when a high data rate service is introduced into the network at the demand of a particular user, the higher transmission power required for this service can lead to a significant and sudden increase in the overall network interference. The closed-loop power control mechanisms in use by other users in the locality tend to react by causing those users in turn to increase their own transmission power, or to request a power increase from their signal provider, in order to maintain the required relevant quality of service. This process is usually non-linear and it may take a considerable amount of time for the network to restore balance following the introduction of such a high data rate service transmission.
Ideally, for each packet transmission in a high data rate channel, the transmission site should use the highest achievable data rate and the minimum transmit power, so that the channel throughput is high, the delay is short and the interference caused to the network is low. Unfortunately, these objectives are very difficult to achieve in practice, primarily due to the following reasons.
Firstly, as mentioned above, high data rate transmissions need high transmit powers. Since certain quality of service must be maintained in all the radio channels, increasing the transmit power for one UE will lead to the increase of transmit powers for many other channels and even other cells.
Secondly, the channel conditions at the UE are very difficult for the BTS to estimate accurately in a timely manner. The channel conditions include the inter-cell interference level, the delay profile and the fading condition of the multipaths. Therefore, the BTS cannot predict accurately the required power level for a chosen data rate to achieve a certain block error rate (BLER) at the UE.
These problems are exacerbated when a substantial increase in the transmission power is needed, such as when starting a very high data rate (e.g. 2 Mb/s) service.
Accordingly, it is desirable to provide a mobile communications network in which the above-mentioned problems are eliminated.